This invention relates generally to limiter circuits and in particular to limiter circuit configurations utilizing an operational amplifier with feedback diodes performing the limiting function. Specifically, the present invention provides an arrangement for compensating the temperature characteristics of the feedback diodes.
Limiter circuits provide, in response to a variable amplitude input signal, an amplitude limited output signal, the amplitude being limited in accordance with some predetermined threshold level. In other words, they provide a constant amplitude output signal as long as an input signal thereto is a predetermined input range. A typical application of a limiter circuit would be to the processing of signals emerging from an IF amplifier or an FM receiver. The limiter circuit removes all amplitude variations from the frequency modulated signal so that a frequency discriminator can operate on a constant amplitude, frequency shifting signal.
Ideally, a limiter should provide its constant output amplitude over a wide temperature range for any input within its design range. A problem associated with the operation of limiters using operational amplifiers with feedback diodes is that the output level, if in limiting, varies with temperature according to the change in the forward voltage drop across the feedback diodes. This output voltage change can be significant over a temperature range of .+-.65.degree. C. Typically, diodes change their forward voltage drop approximately -2.1 mV/.degree.C. When a typical output voltage level is set to about 0.9 volts RMS or approximately 2.5 volts peak-to-peak, the 273 mV change in the diode's forward voltage drop is a significant factor over the wide temperature range.
In order to minimize the change in forward voltage drop with temperature, a known compensating arrangement utilizes a resistive voltage divider to provide a predetermined level of back bias on the feedback diodes around the operational amplifier. This resistive voltage divider includes only very close tolerance resistors. The back bias is quite stable over temperature and the diode forward bias potential change is not compensated for in the network. The problem with such an arrangement is that the AC signal from the limiter clips and limits at different levels as the temperature changes.
Another approach to temperature compensating limiter circuits is shown in U.S. Pat. No. 2,927,223--Meirowitz (issued Mar. 1, 1960). This patent is specifically incorporated herein by reference thereto.
Referring to FIG. 2 of the Meirowitz patent, there is shown diodes 18 and 19 intended to provide temperature compensation to the limiting diodes 11 and 12. This circuit, however, is not very practical for many applications because it requires two separate and distinct power supplies, namely source V.sub.b and voltage source 20. In addition, it provides no amplification of lower level signals. Furthermore, the Meirowitz arrangement requires that all used diodes have substantially similar conduction (current) characteristics over the temperature range. Also, it is necessary to select resistor 21 in order to set the current level through compensating diodes 18 and 19, which in turn sets the forward voltage drop across them.